1. Field of the Invention
The present invention relates in general to electrolytic solutions and, more particularly, to an electrolytic solution for use as a gel electrolyte in an electrolytic cell.
2. Background Art
Electrolytic solutions have been used in forming cured or thermoset gel electrolytes for use in electrolytic cells for several years. In particular, polyethylene oxide (PEO) has been used in combination with other electrolyte materials such as propylene carbonate to form thermoset gel polymer systems. The use of polyethylene oxide has been particularly useful for, among other things, increasing the viscosity of an electrolyte solution to, in turn, improve coating and flow properties on an associated substrate, such as an electrode, before curing the electrolyte solution to a thermoset gel.
Although polyethylene oxide has been used with some degree of success, electrolytic solutions incorporating PEO have had certain drawbacks, such as thermodynamic instability. This instability leads to inadvertent precipitation and crystallization from the solution before intended curing which, in turn, results in non-uniform coatings and subsequent difficulties with controlling coating of the electrolytic solution onto a substrate as well as inhomogeneities of ionic conductivity. Furthermore, the use of PEO has resulted in the formation of thermoset gel electrolytes which lack desired mechanical properties. In particular, these gel electrolytes exhibit relatively low compressive strengths, and a relatively high compressive modulus.
Another type of prior art gel polymer system incorporates poly(methyl methacrylate) (PMMA) into a thermoplastic gel electrolyte. For instance, "Fast Ion Transport in New Lithium Electrolytes Gelled with PMMA", Solid State Ionics 66, pp. 97-104 (1993) by O. Bohnke, et al. disclosed, the use of 30-35 wt. % PMMA to form a thermoplastic electrolytic gel. Moreover, in "Ionic Conductivity and Compatibility Studies of Blends of Poly(methyl Methacrylate) and Poly(propylene Glycol) Complexed with LiCH.sub.3 SO.sub.3 ", Journal of Polymer Science, Vol. 30, pp. 2025-2031, J. R. Stevens et al. contemplated the polymerization of PMMA to form a thermoplastic gel electrolyte. Finally, in "Conductivity and Viscosity Studies of Lithium Ion Conductive Electrolytes Gelled with Poly(methyl methacrylate)", Advance Materials Research (1994), M. Rezrazi et al. used PMMA to form a gel in a liquid electrolyte system.
Although these references disclose the use of PMMA in formation of an electrolytic solution and, in turn, a thermoplastic electrolyte gel, these gel polymer systems either (a) use ranges of PMMA that result in a thermoplastic gel when the PMMA is mixed with propylene carbonate, or (b) use PMMA to form a portion of the thermoplastic gel structure in a gel polymer system. Furthermore, not withstanding the less than desirable mechanical properties of the PMMA based electrolytes, the PMMA in such prior art devices is used as the actual gelling agent, which, in turn, results in a thermoplastic gel.
Accordingly, it is an object of the present invention to provide an electrolytic solution which incorporates PMMA into the electrolyte composition as a reinforcement polymer to increase the mechanical integrity of the resulting thermoset gel electrolyte. It is another object of the present invention to provide an electrolyte having PMMA remaining in solution after curing of the electrolyte.
It is still further an object of the present invention to provide an electrolytic solution which is thermodynamically stable.
It is yet another object of the present invention to provide an electrolytic solution which will facilitate an uniform, homogenous solution having increased coatability and adhesion onto an associated substrate. These and other objects will become apparent in light of the present Specification, Claims and Drawings.